Add asset metadata, sharing, and MinIO-backed signed links.

This introduces deduplicated per-user image/3D asset records linked into feature properties, adds visibility-controlled download routing, and wires local S3-compatible storage with automatic bucket bootstrap in Docker Compose.

Made-with: Cursor

docs/docker-minio-local-dev.md

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+# Docker MinIO Local Development
+
+Local object storage is provided by MinIO in `docker-compose.yml`.
+
+## Port policy
+
+- MinIO S3 API (`9000`) is internal-only (not published on host).
+- MinIO admin UI is exposed on `8774`.
+
+## Services
+
+- `minio`: object storage
+- `minio-init`: one-shot bucket bootstrap using `mc`
+- `api` / `api-dev`: use MinIO via internal DNS endpoint `minio:9000`
+
+## Environment variables
+
+- `S3_ENDPOINT` (default `minio:9000`)
+- `S3_BUCKET` (default `momswap-assets`)
+- `S3_REGION` (default `us-east-1`)
+- `S3_ACCESS_KEY` (default `momswap`)
+- `S3_SECRET_KEY` (default `momswap-secret`)
+- `S3_USE_PATH_STYLE` (default `true`)
+- `S3_USE_TLS` (default `false`)
+
+## Start stack
+
+```bash
+./bin/gen-server-keys.sh
+docker compose up --build -d
+```
+
+## Verify storage setup
+
+1. Confirm only MinIO UI is published:
+   ```bash
+   docker compose ps
+   ```
+2. Open MinIO admin console:
+   - `http://localhost:8774`
+3. Confirm bucket exists (`momswap-assets` by default).
+4. Use API flow:
+   - create asset and get signed upload URL
+   - upload file with PUT
+   - request `/v1/assets/{id}/download`